diff --git a/src/main/java/com/google/devtools/build/lib/analysis/BUILD b/src/main/java/com/google/devtools/build/lib/analysis/BUILD
index 598d754..ca009b0 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/BUILD
+++ b/src/main/java/com/google/devtools/build/lib/analysis/BUILD
@@ -1767,7 +1767,9 @@
         "//src/main/java/com/google/devtools/build/lib/starlarkbuildapi/config",
         "//src/main/java/net/starlark/java/eval",
         "//src/main/java/net/starlark/java/syntax",
+        "//third_party:error_prone_annotations",
         "//third_party:guava",
+        "//third_party:jsr305",
     ],
 )
 
@@ -2094,7 +2096,10 @@
         "//src/main/java/com/google/devtools/build/lib/util",
         "//src/main/java/com/google/devtools/common/options",
         "//src/main/java/net/starlark/java/eval",
+        "//src/main/java/net/starlark/java/syntax",
+        "//third_party:error_prone_annotations",
         "//third_party:guava",
+        "//third_party:jsr305",
     ],
 )
 
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/config/StarlarkDefinedConfigTransition.java b/src/main/java/com/google/devtools/build/lib/analysis/config/StarlarkDefinedConfigTransition.java
index caf5a40..031f046 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/config/StarlarkDefinedConfigTransition.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/config/StarlarkDefinedConfigTransition.java
@@ -23,9 +23,11 @@
 import com.google.devtools.build.lib.packages.BazelStarlarkContext;
 import com.google.devtools.build.lib.packages.StructImpl;
 import com.google.devtools.build.lib.starlarkbuildapi.config.ConfigurationTransitionApi;
+import com.google.errorprone.annotations.FormatMethod;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
+import javax.annotation.Nullable;
 import net.starlark.java.eval.Dict;
 import net.starlark.java.eval.EvalException;
 import net.starlark.java.eval.Mutability;
@@ -109,13 +111,14 @@
    *     child configuration (split transitions will have multiple elements in this map with keys
    *     provided by the transition impl, patch transitions should have a single element keyed by
    *     {@code PATCH_TRANSITION_KEY}). Each build setting map is a map from build setting to target
-   *     setting value; all other build settings will remain unchanged
-   * @throws EvalException if there is an error evaluating the transition
+   *     setting value; all other build settings will remain unchanged. Returns null if errors were
+   *     reported to the handler.
    * @throws InterruptedException if evaluating the transition is interrupted
    */
+  @Nullable
   public abstract ImmutableMap<String, Map<String, Object>> evaluate(
       Map<String, Object> previousSettings, StructImpl attributeMap, EventHandler eventHandler)
-      throws EvalException, InterruptedException;
+      throws InterruptedException;
 
   public static StarlarkDefinedConfigTransition newRegularTransition(
       StarlarkCallable impl,
@@ -218,18 +221,23 @@
      * @param attributeMapper a map of attributes
      */
     // TODO(bazel-team): integrate dict-of-dicts return type with ctx.split_attr
+    @Nullable
     @Override
     public ImmutableMap<String, Map<String, Object>> evaluate(
-        Map<String, Object> previousSettings, StructImpl attributeMapper, EventHandler eventHandler)
-        throws EvalException, InterruptedException {
+        Map<String, Object> previousSettings, StructImpl attributeMapper, EventHandler handler)
+        throws InterruptedException {
+      // Call the Starlark function.
       Object result;
-      try {
+      try (Mutability mu = Mutability.create("eval_transition_function")) {
+        StarlarkThread thread = new StarlarkThread(mu, semantics);
+        thread.setPrintHandler(Event.makeDebugPrintHandler(handler));
+        starlarkContext.storeInThread(thread);
         result =
-            evalFunction(impl, ImmutableList.of(previousSettings, attributeMapper), eventHandler);
-      } catch (EvalException e) {
-        // TODO(adonovan): this doesn't look right. Consider interposing a call to a delegating
-        // wrapper just to establish a fake frame for impl, then remove the catch.
-        throw new EvalException(impl.getLocation(), e.getMessageWithStack());
+            Starlark.fastcall(
+                thread, impl, new Object[] {previousSettings, attributeMapper}, new Object[0]);
+      } catch (EvalException ex) {
+        handler.handle(Event.error(null, ex.getMessageWithStack()));
+        return null;
       }
 
       if (result instanceof Dict) {
@@ -260,9 +268,12 @@
           return ImmutableMap.of(
               PATCH_TRANSITION_KEY,
               Dict.cast(result, String.class, Object.class, "dictionary of options"));
-        } catch (EvalException e) {
-          throw new EvalException(impl.getLocation(), e.getMessage());
+        } catch (EvalException ex) {
+          // TODO(adonovan): explain "want dict<string, any> or dict<string, dict<string, any>>".
+          errorf(handler, "invalid result from transition function: %s", ex.getMessage());
+          return null;
         }
+
       } else if (result instanceof Sequence) {
         ImmutableMap.Builder<String, Map<String, Object>> builder = ImmutableMap.builder();
         try {
@@ -274,34 +285,32 @@
                 Integer.toString(i++),
                 Dict.cast(toOptions, String.class, Object.class, "dictionary of options"));
           }
-        } catch (EvalException e) {
-          throw new EvalException(impl.getLocation(), e.getMessage());
+        } catch (EvalException ex) {
+          // TODO(adonovan): explain "want sequence of dict<string, any>".
+          errorf(handler, "invalid result from transition function: %s", ex.getMessage());
+          return null;
         }
         return builder.build();
+
       } else {
-        throw new EvalException(
-            impl.getLocation(),
-            "Transition function must return a dictionary or list of dictionaries.");
+        errorf(
+            handler,
+            "transition function returned %s, want dict or list of dicts",
+            Starlark.type(result));
+        return null;
       }
     }
 
+    @FormatMethod
+    private void errorf(EventHandler handler, String format, Object... args) {
+      handler.handle(Event.error(impl.getLocation(), String.format(format, args)));
+    }
+
     @Override
     public void repr(Printer printer) {
       printer.append("<transition object>");
     }
 
-    /** Evaluate the input function with the given argument, and return the return value. */
-    private Object evalFunction(
-        StarlarkCallable function, ImmutableList<Object> args, EventHandler eventHandler)
-        throws InterruptedException, EvalException {
-      try (Mutability mu = Mutability.create("eval_transition_function")) {
-        StarlarkThread thread = new StarlarkThread(mu, semantics);
-        thread.setPrintHandler(Event.makeDebugPrintHandler(eventHandler));
-        starlarkContext.storeInThread(thread);
-        return Starlark.call(thread, function, args, /*kwargs=*/ ImmutableMap.of());
-      }
-    }
-
     @Override
     public boolean equals(Object object) {
       if (object == this) {
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/starlark/FunctionTransitionUtil.java b/src/main/java/com/google/devtools/build/lib/analysis/starlark/FunctionTransitionUtil.java
index f68f9ac..75c24db 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/starlark/FunctionTransitionUtil.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/starlark/FunctionTransitionUtil.java
@@ -27,12 +27,14 @@
 import com.google.devtools.build.lib.analysis.config.FragmentOptions;
 import com.google.devtools.build.lib.analysis.config.StarlarkDefinedConfigTransition;
 import com.google.devtools.build.lib.cmdline.Label;
+import com.google.devtools.build.lib.events.Event;
 import com.google.devtools.build.lib.events.EventHandler;
 import com.google.devtools.build.lib.packages.StructImpl;
 import com.google.devtools.build.lib.util.Fingerprint;
 import com.google.devtools.common.options.OptionDefinition;
 import com.google.devtools.common.options.OptionsParser;
 import com.google.devtools.common.options.OptionsParsingException;
+import com.google.errorprone.annotations.FormatMethod;
 import java.lang.reflect.Field;
 import java.util.Collection;
 import java.util.HashSet;
@@ -42,6 +44,7 @@
 import java.util.Set;
 import java.util.TreeMap;
 import java.util.TreeSet;
+import javax.annotation.Nullable;
 import net.starlark.java.eval.Dict;
 import net.starlark.java.eval.EvalException;
 import net.starlark.java.eval.Mutability;
@@ -68,34 +71,57 @@
    * @param buildOptions the pre-transition build options
    * @param starlarkTransition the transition to apply
    * @param attrObject the attributes of the rule to which this transition is attached
-   * @return the post-transition build options.
+   * @return the post-transition build options, or null if errors were reported to handler.
    */
+  @Nullable
   static Map<String, BuildOptions> applyAndValidate(
       BuildOptions buildOptions,
       StarlarkDefinedConfigTransition starlarkTransition,
       StructImpl attrObject,
-      EventHandler eventHandler)
-      throws EvalException, InterruptedException {
-    checkForDenylistedOptions(starlarkTransition);
+      EventHandler handler)
+      throws InterruptedException {
+    try {
+      checkForDenylistedOptions(starlarkTransition);
 
-    // TODO(waltl): consider building this once and use it across different split
-    // transitions.
-    Map<String, OptionInfo> optionInfoMap = buildOptionInfo(buildOptions);
-    Dict<String, Object> settings = buildSettings(buildOptions, optionInfoMap, starlarkTransition);
+      // TODO(waltl): consider building this once and use it across different split
+      // transitions.
+      Map<String, OptionInfo> optionInfoMap = buildOptionInfo(buildOptions);
+      Dict<String, Object> settings =
+          buildSettings(buildOptions, optionInfoMap, starlarkTransition);
 
-    ImmutableMap.Builder<String, BuildOptions> splitBuildOptions = ImmutableMap.builder();
+      ImmutableMap.Builder<String, BuildOptions> splitBuildOptions = ImmutableMap.builder();
 
-    ImmutableMap<String, Map<String, Object>> transitions =
-        starlarkTransition.evaluate(settings, attrObject, eventHandler);
-    validateFunctionOutputsMatchesDeclaredOutputs(transitions.values(), starlarkTransition);
+      ImmutableMap<String, Map<String, Object>> transitions =
+          starlarkTransition.evaluate(settings, attrObject, handler);
+      if (transitions == null) {
+        return null; // errors reported to handler
+      }
+      validateFunctionOutputsMatchesDeclaredOutputs(transitions.values(), starlarkTransition);
 
-    for (Map.Entry<String, Map<String, Object>> entry : transitions.entrySet()) {
-      Map<String, Object> newValues = handleImplicitPlatformChange(entry.getValue());
-      BuildOptions transitionedOptions =
-          applyTransition(buildOptions, newValues, optionInfoMap, starlarkTransition);
-      splitBuildOptions.put(entry.getKey(), transitionedOptions);
+      for (Map.Entry<String, Map<String, Object>> entry : transitions.entrySet()) {
+        Map<String, Object> newValues = handleImplicitPlatformChange(entry.getValue());
+        BuildOptions transitionedOptions =
+            applyTransition(buildOptions, newValues, optionInfoMap, starlarkTransition);
+        splitBuildOptions.put(entry.getKey(), transitionedOptions);
+      }
+      return splitBuildOptions.build();
+
+    } catch (ValidationException ex) {
+      handler.handle(
+          Event.error(starlarkTransition.getLocationForErrorReporting(), ex.getMessage()));
+      return null;
     }
-    return splitBuildOptions.build();
+  }
+
+  private static final class ValidationException extends Exception {
+    ValidationException(String message) {
+      super(message);
+    }
+
+    @FormatMethod
+    static ValidationException format(String format, Object... args) {
+      return new ValidationException(String.format(format, args));
+    }
   }
 
   /**
@@ -110,7 +136,7 @@
    *   <li>Result: the mapping accidentally overrides the transition
    * </ol>
    *
-   * <p>Transitions can alo explicitly set --platforms to be clear what platform they set.
+   * <p>Transitions can also explicitly set --platforms to be clear what platform they set.
    *
    * <p>Platform mappings:
    * https://docs.bazel.build/versions/master/platforms-intro.html#platform-mappings.
@@ -132,9 +158,9 @@
   }
 
   private static void checkForDenylistedOptions(StarlarkDefinedConfigTransition transition)
-      throws EvalException {
+      throws ValidationException {
     if (transition.getOutputs().contains("//command_line_option:define")) {
-      throw Starlark.errorf(
+      throw new ValidationException(
           "Starlark transition on --define not supported - try using build settings"
               + " (https://docs.bazel.build/skylark/config.html#user-defined-build-settings).");
     }
@@ -148,18 +174,19 @@
   private static void validateFunctionOutputsMatchesDeclaredOutputs(
       Collection<Map<String, Object>> transitions,
       StarlarkDefinedConfigTransition starlarkTransition)
-      throws EvalException {
+      throws ValidationException {
     for (Map<String, Object> transition : transitions) {
       LinkedHashSet<String> remainingOutputs =
           Sets.newLinkedHashSet(starlarkTransition.getOutputs());
       for (String outputKey : transition.keySet()) {
         if (!remainingOutputs.remove(outputKey)) {
-          throw Starlark.errorf("transition function returned undeclared output '%s'", outputKey);
+          throw ValidationException.format(
+              "transition function returned undeclared output '%s'", outputKey);
         }
       }
 
       if (!remainingOutputs.isEmpty()) {
-        throw Starlark.errorf(
+        throw ValidationException.format(
             "transition outputs [%s] were not defined by transition function",
             Joiner.on(", ").join(remainingOutputs));
       }
@@ -195,14 +222,14 @@
    * @throws RuntimeException If the field corresponding to an option value in buildOptions is
    *     inaccessible due to Java language access control, or if an option name is an invalid key to
    *     the Starlark dictionary
-   * @throws EvalException if any of the specified transition inputs do not correspond to a valid
-   *     build setting
+   * @throws ValidationException if any of the specified transition inputs do not correspond to a
+   *     valid build setting
    */
   static Dict<String, Object> buildSettings(
       BuildOptions buildOptions,
       Map<String, OptionInfo> optionInfoMap,
       StarlarkDefinedConfigTransition starlarkTransition)
-      throws EvalException {
+      throws ValidationException {
     LinkedHashSet<String> remainingInputs = Sets.newLinkedHashSet(starlarkTransition.getInputs());
 
     try (Mutability mutability = Mutability.create("build_settings")) {
@@ -219,15 +246,16 @@
         }
         OptionInfo optionInfo = entry.getValue();
 
+        Field field = optionInfo.getDefinition().getField();
+        FragmentOptions options = buildOptions.get(optionInfo.getOptionClass());
         try {
-          Field field = optionInfo.getDefinition().getField();
-          FragmentOptions options = buildOptions.get(optionInfo.getOptionClass());
           Object optionValue = field.get(options);
-
           dict.putEntry(optionKey, optionValue == null ? Starlark.NONE : optionValue);
         } catch (IllegalAccessException e) {
           // These exceptions should not happen, but if they do, throw a RuntimeException.
           throw new RuntimeException(e);
+        } catch (EvalException ex) {
+          throw new IllegalStateException(ex); // can't happen
         }
       }
 
@@ -236,11 +264,15 @@
         if (!remainingInputs.remove(starlarkOption.getKey().toString())) {
           continue;
         }
-        dict.putEntry(starlarkOption.getKey().toString(), starlarkOption.getValue());
+        try {
+          dict.putEntry(starlarkOption.getKey().toString(), starlarkOption.getValue());
+        } catch (EvalException ex) {
+          throw new IllegalStateException(ex); // can't happen
+        }
       }
 
       if (!remainingInputs.isEmpty()) {
-        throw Starlark.errorf(
+        throw ValidationException.format(
             "transition inputs [%s] do not correspond to valid settings",
             Joiner.on(", ").join(remainingInputs));
       }
@@ -261,14 +293,14 @@
    * @param starlarkTransition transition object that is being applied. Used for error reporting and
    *     checking for analysis testing
    * @return the post-transition build options
-   * @throws EvalException If a requested option field is inaccessible
+   * @throws ValidationException If a requested option field is inaccessible
    */
   private static BuildOptions applyTransition(
       BuildOptions buildOptionsToTransition,
       Map<String, Object> newValues,
       Map<String, OptionInfo> optionInfoMap,
       StarlarkDefinedConfigTransition starlarkTransition)
-      throws EvalException {
+      throws ValidationException {
     BuildOptions buildOptions = buildOptionsToTransition.clone();
     // The names and values of options that are different after this transition.
     Set<String> convertedNewValues = new HashSet<>();
@@ -301,7 +333,7 @@
         }
         try {
           if (!optionInfoMap.containsKey(optionName)) {
-            throw Starlark.errorf(
+            throw ValidationException.format(
                 "transition output '%s' does not correspond to a valid setting", entry.getKey());
           }
 
@@ -340,7 +372,7 @@
           } else if (optionValue instanceof String) {
             convertedValue = def.getConverter().convert((String) optionValue);
           } else {
-            throw Starlark.errorf("Invalid value type for option '%s'", optionName);
+            throw ValidationException.format("Invalid value type for option '%s'", optionName);
           }
 
           Object oldValue = field.get(options);
@@ -352,13 +384,13 @@
           }
 
         } catch (IllegalArgumentException e) {
-          throw Starlark.errorf(
+          throw ValidationException.format(
               "IllegalArgumentError for option '%s': %s", optionName, e.getMessage());
         } catch (IllegalAccessException e) {
           throw new RuntimeException(
               "IllegalAccess for option " + optionName + ": " + e.getMessage());
         } catch (OptionsParsingException e) {
-          throw Starlark.errorf(
+          throw ValidationException.format(
               "OptionsParsingError for option '%s': %s", optionName, e.getMessage());
         }
       }
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/starlark/StarlarkAttributeTransitionProvider.java b/src/main/java/com/google/devtools/build/lib/analysis/starlark/StarlarkAttributeTransitionProvider.java
index b086aca..d207a26 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/starlark/StarlarkAttributeTransitionProvider.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/starlark/StarlarkAttributeTransitionProvider.java
@@ -25,7 +25,6 @@
 import com.google.devtools.build.lib.analysis.config.StarlarkDefinedConfigTransition;
 import com.google.devtools.build.lib.analysis.config.transitions.SplitTransition;
 import com.google.devtools.build.lib.analysis.config.transitions.TransitionFactory;
-import com.google.devtools.build.lib.events.Event;
 import com.google.devtools.build.lib.events.EventHandler;
 import com.google.devtools.build.lib.packages.Attribute;
 import com.google.devtools.build.lib.packages.AttributeMap;
@@ -36,7 +35,6 @@
 import com.google.devtools.build.lib.starlarkbuildapi.SplitTransitionProviderApi;
 import java.util.LinkedHashMap;
 import java.util.Map;
-import net.starlark.java.eval.EvalException;
 import net.starlark.java.eval.Printer;
 
 /**
@@ -109,16 +107,12 @@
       // outputs. Rather than complicate BuildOptionsView with more access points to BuildOptions,
       // we just use the original BuildOptions and trust the transition's enforcement logic.
       BuildOptions buildOptions = buildOptionsView.underlying();
-      try {
-        return applyAndValidate(
-            buildOptions, starlarkDefinedConfigTransition, attrObject, eventHandler);
-      } catch (EvalException e) {
-        eventHandler.handle(
-            Event.error(
-                starlarkDefinedConfigTransition.getLocationForErrorReporting(),
-                e.getMessageWithStack()));
+      Map<String, BuildOptions> res =
+          applyAndValidate(buildOptions, starlarkDefinedConfigTransition, attrObject, eventHandler);
+      if (res == null) {
         return ImmutableMap.of("error", buildOptions.clone());
       }
+      return res;
     }
   }
 }
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/starlark/StarlarkRuleTransitionProvider.java b/src/main/java/com/google/devtools/build/lib/analysis/starlark/StarlarkRuleTransitionProvider.java
index a0f4de2..259ede2 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/starlark/StarlarkRuleTransitionProvider.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/starlark/StarlarkRuleTransitionProvider.java
@@ -34,7 +34,6 @@
 import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.Objects;
-import net.starlark.java.eval.EvalException;
 
 /**
  * This class implements {@link TransitionFactory} to provide a starlark-defined transition that
@@ -108,20 +107,13 @@
     @Override
     public BuildOptions patch(BuildOptionsView buildOptionsView, EventHandler eventHandler)
         throws InterruptedException {
-      Map<String, BuildOptions> result;
       // Starlark transitions already have logic to enforce they only access declared inputs and
       // outputs. Rather than complicate BuildOptionsView with more access points to BuildOptions,
       // we just use the original BuildOptions and trust the transition's enforcement logic.
       BuildOptions buildOptions = buildOptionsView.underlying();
-      try {
-        result =
-            applyAndValidate(
-                buildOptions, starlarkDefinedConfigTransition, attrObject, eventHandler);
-      } catch (EvalException e) {
-        eventHandler.handle(
-            Event.error(
-                starlarkDefinedConfigTransition.getLocationForErrorReporting(),
-                e.getMessageWithStack()));
+      Map<String, BuildOptions> result =
+          applyAndValidate(buildOptions, starlarkDefinedConfigTransition, attrObject, eventHandler);
+      if (result == null) {
         return buildOptions.clone();
       }
       if (result.size() != 1) {
diff --git a/src/test/java/com/google/devtools/build/lib/analysis/StarlarkRuleTransitionProviderTest.java b/src/test/java/com/google/devtools/build/lib/analysis/StarlarkRuleTransitionProviderTest.java
index bb66659..d4ddb73 100644
--- a/src/test/java/com/google/devtools/build/lib/analysis/StarlarkRuleTransitionProviderTest.java
+++ b/src/test/java/com/google/devtools/build/lib/analysis/StarlarkRuleTransitionProviderTest.java
@@ -133,7 +133,7 @@
 
     reporter.removeHandler(failFastHandler);
     getConfiguredTarget("//test");
-    assertContainsEvent("Transition function must return a dictionary or list of dictionaries.");
+    assertContainsEvent("transition function returned string, want dict or list of dicts");
   }
 
   @Test